This isn't a support request, so moved it here.
GNOME 2 to GNOME 3 is a desktop paradigm shift. I don't think the same, or at least to the same extent, can said about KDE 3 to KDE 4. The reason to develop Cinnamon (
history) and to contribute to MATE is that those, along with Xfce, allowed the Linux Mint developers to continue the paradigm that aligns with their vision for the desktop (very short: panel at bottom of the screen with menu button, launcher buttons, task list, and status icons). The same paradigm is possible with KDE, so from that view there is no need to backtrack to KDE 3 to implement the Linux Mint developers' vision for the desktop with KDE.
If you want to you can install Trinity (KDE 3) on your Linux Mint system by following the steps for Ubuntu 14.04 LTS (assuming you are using Linux Mint 17.x):
https://wiki.trinitydesktop.org/Ubuntu_ ... structions
Also, a poll was done in 2012 to get feedback from the community of users on which desktop environments to support on Linux Mint forward:
http://forums.linuxmint.com/viewtopic.p ... 3&t=100787. As you can see Trinity got the least amount of votes from users, while KDE was in the top 4. It's hard to justify developers spending time on a desktop environment that less than 0.5% of the users (in the poll) say they want the developers to their spend time on.
That said, if there is sufficient interest today, and there are capable volunteers to do a community edition "Linux Mint Trinity", that might be something that could be worked out with the Linux Mint developers (community editions mean you can use the Linux Mint name for the distribution, and the ISOs will be tested by Linux Mint's testing team to assure the level of quality). Maintaining a distribution is a lot of work and takes commitment.
